Creatine transporter deficiency (CRTR-D) is an X-hnked disorder characterized by hypotonia, developmental delay, and seizures. We report the third Japanese family with CRTR-D. The proband was an 8-year-old boy who presented with hypotonia, severe intellectual disability and two episodes of seizures associated with/without fever Among 7 siblings (4 males, 3 females), the eldest brother had severe intellectual disability, epilepsy, and sudden death at 17 years of age, while 18-year-old third elder brother had severe intellectual disability, autism, and drug-resistant epilepsy The proband's urinary creatine/creatinine ratio was increased A reduced creatine peak on brain magnetic resonance spectroscopy and a known pathogenic mutation in the SLC6A8 gene (c.1661 C&gt
T,p.Pro554Leu) confirmed the diagnosis of CRTR-D The same mutation was found in the third elder brother. Their mother was a heterozygote. Symptoms of CRTR-D are non-specific. Urinary creatine/creatinine ratio should be measured in patients with hypotonia, developmental delay, seizure and autism whose family history indicates an X-linked inheritance.